Multiwall pipe is suitable for pipeline applications when single-wall pipe is not acceptable due to environmental, safety and thermal considerations, as well as negative buoyancy. Sections of multiwall pipe are typically joined to form pipeline systems which provide an effective means of containing the material being transported. For example, multiwall pipe is typically used for transporting corrosive or hazardous materials, such as oil, for slurry pipe lines, and for insulated sub-sea lines.
Multiwall pipe typically comprises a steel outer casing and an inner liner pipe. The outer casing provides tensile strength to the pipeline, as well as secondary containment of the material being transported. The inner liner pipe provides a corrosion barrier, as well as radial strength. The annulus between the two casings and the liner may be filled with an incompressible cement grout insulating material in order to provide thermal protection for transporting temperature-sensitive materials, such as crude oil or heavy fuel oil, when other insulating methods are unsuitable.
Presently, sections of multiwall pipe are joined in two steps: first, the inner liner pipe layer is joined, followed by welding together the sections of outer casing. The sections of inner liner pipe, which can be made from materials such as steel, fiberglass, polyethylene, or polyvinyl chloride (PVC), are joined in several ways, including full circumferential welds, interference fit bands, or butt wrap joints. The steel casing is then joined by full circumferential welds. Having to perform these two operations in order to join sections of multiwall pipe is both time-consuming and expensive. Further, using such methods with sections of large-diameter multi-wall pipe has had a low success rate.
The disadvantages of the prior art are overcome by the present invention. Novel methods and apparatus are hereinafter described for efficiently and reliably joining sections of multiwall pipe, even large diameter pipe.